1. Field of the Invention
The present invention relates to materials for organic electroluminescence devices and organic electroluminescence devices using the materials and, more particularly to materials for organic electroluminescence devices which realize electroluminescence devices exhibiting a high emitting efficiency, causing little pixel defects, exhibiting a high heat resistance and having a long lifetime.
2. Description of the Prior Art
An organic electroluminescence device (organic ELdevice) is a spontaneous emission device which utilizes the phenomenon of fluorescence which occurs by the energy of recombination between holes injected from an anode and electrons injected from a cathode by application of electric field. As the structure of organic EL devices, a two-layered structure having a hole transporting (injecting) layer and an electron transporting/light emitting layer and a three-layered structure having a hole transporting (injecting) layer, a light emitting layer and an electron transporting (injecting) layer are well known. To increase the efficiency of recombination of injected holes and electrons in laminated devices, the structure of the device and the process for forming the device have been studied
It is recently proposed to use a light emitting layer of a phosphorescent material in addition to a light emitting layer of a fluorescent material. A high efficiency of light emission is achieved by utilizing the excited singlet state and the excited triplet state of the organic phosphorescent material in the light emitting layer. It is considered that the singlet exciton and the triplet exciton are formed in a proportion of 1:3 due to the difference in the spin multiplicity when electrons and holes are recombined in an organic EL device. Therefore, it is expected that an efficiency of light emission 3 to 4 times as great as that of a device utilizing only the fluorescent material can be achieved by utilizing a phosphorescent material.
To prevent the excited triplet state or the triplet exciton from quenching, the organic EL devices described above are made into a laminate structure having an anode, a hole transporting layer, an organic light emitting layer, an electron transporting layer (a hole blocking layer), an electron transporting layer and a cathode in this order, while using an organic light emitting layer made of a host compound and a phosphorescent compound (for example, Patent Documents 1-6). In these Patent Documents, host materials having a dibenzofuran structure or a dibenzothiophene structure are described. However, there is nothing about their advantage in the device performace as compared with a host material having a carbazole structure.
Patent Documents 7 and 8 disclose compounds prepared by bonding a carbazole structure to dibenzo compounds. The proposed compounds are used in a host material for a blue phosphorescent device in examples thereof. The compounds of the present invention are not taught and the effect is uncertain.
Patent Documents 9, 10 and 11 disclose compounds prepared by bonding an anthracene structure to dibenzo compounds. However, the energy level of the excited triplet state of the proposed compounds is small because of the anthracene structure. Therefore, a blue phosphorescent material does not emit light even if any of the proposed compounds is used as a host material for the light emitting layer.
Patent Document 12 discloses dibenzofuran compounds essentially having at least two polymerizable functional groups. If the host material of a phosphorescent device has in its molecule a polymerizable group such as propenylene group, vinylene group and 4-propyl-2-pentenylene group which is formed by introducing an unsaturated bond such as double bond and triple bond, radicals are propagatedly generated in the device to adversely affect the emitting efficiency and lifetime.    [Patent Document 1] WO 05/101912    [Patent Document 2] JP 5-109485A    [Patent Document 3] JP 2004-002351A    [Patent Document 4] WO 041096945    [Patent Document 5] JP 2002-308837A    [Patent Document 6] WO 2005-113531    [Patent Document 7] JP 2005-112765A    [Patent Document 8] WO 2006-114966    [Patent Document 9] JP 2005-314239A    [Patent Document 10] JP 2007-77094A    [Patent Document 11] JP 2007-63501A    [Patent Document 12] JP 2007-110097A